15% would be huge.
Particularly if combined with a low black turnout,15% would be huge.
Two black voters deciding not to vote at all is like one black voter voting for Trump.. . . and as you say, a 15% vote of blacks for Trump makes unattainable victories possible.
My fond hope, and IMHO Trump’s as well, is for black voters to cement Trump's victory and to swing the House and keep the Senate Republican.
I believe it has always been a major objective of Trump to make the Republican Party competitive for the black vote. You want to talk about legacies! IMHO that would be bigger than making ACB a SCOTUS justice. And Donald J. Trump doesn’t think small.
But the other critical need of the polity is to overturn the Warren Court’s unanimous 1964 New York Times Co. v. Sullivan decision, restoring the status quo ante in which it was actually illegal to libel Republican candidates and office holders (Democrats never get libeled). And Trump has said as much, with Levin’s concurrence.